What do you mean by sub c?

If a car of mass m goes round a corner at velocity v with radius r, it is the frictional force which provides the centripetal force and you get:

\mu R = \frac{mv^2}{r}

Since R = mg

\mu mg = \frac{mv^2}{r}

\mu g = \frac{v^2}{r}

Solve for v.